Self-propagating High-temperature Synthesis Of Al-based Intermetallics And Study Of The Reaction Activity

The combustion synthesis process and production of Al-based intermetallics composite materials,from compacts composed of Ni-Al,Ti-Al,Ti-Al and the additives(including Ni2O3,Fe3O4,MnO2,B,C,Si),was researched by self-propagating high-temperature synthesis(SHS)which is motivated by ignition charge.The theoretical calculation of the standard formation enthalpy of reaction system and the adiabatic combustion temperature was proceeded.The influence of the experimental factors on the combustion synthesis process and combustion synthesis product was studied,then matching and verifing the model and equation the of combustion process.(1)We explored the influence on the standard formation enthalpy and adiabatic combustion temperature of reaction system which was affected by the change of the ratio of reactants and additives content.Through the theoretical calculation,in Ni-Al,Ti-Al systemes only when the molar ratio of Ni:Al is 1:1,Ti:Al is 1:1 and 3:1 can occur SHS,the other ratios of different reactant powders cannot exceed the adiabatic combustion temperature of 1800K.The additive of Ni2O3,Fe3O4,MnO2,B,C,and Si can not only enhance the heat released from the combustion,but also increase the adiabatic combustion temperature of the compacts.The DSC experiment and the calculation of the standard formation gibbs free energy for different reaction system,which identified the first reaction is aluminothermic reduction reaction and eutectic reaction between A1 and non-metallic elemental,and the most stable product were TiB2,TiC,Ti5Si3 and Al2O3.(2)In Ni-Al,Ti-Al reaction system,the weight of the compacts,the ratio of reactants,particle size and compress pressure of the reactant were studied,revealing the influences on the combustion process and products.Combustion temperature is increased with augment of the weight to compact,but density and porosity of the product do not changed distinctly.Only when the mole ratio of Ni:Al is 1:1,and Ti:Al is 1:1 or 3:1,the combustion system can occur self-propagating high-temperature synthesis.And the change of the combustion temperature in line with the previous theoretical calculation,but using 200 mesh Ni powder cannot occur self-propagating high-temperature synthesis.The combustion temperature,combustion rate and the highest heating rate of the compact are increased when fine powders were used,with the porosity and volume expansion of the product was descent.With the decline of the compaction pressure,the density of the primary compacts,combustion temperature,combustion rate and the highest heating rate are all decreased,and the porosity and volume expansion of the product are ascend.Results of XRD and SEM showing that the porous intermetallic compounds is generated.(3)By changing the metal oxide and nonmetal elemental contents,we revealed the influence of additive contents on the combustion synthesis process and products.The density of the primary compact is increased with the metallic oxide added,but decreased with the addition elementary substance.The addition of the metallic oxide(Ni2O3,Fe3O4,MnO2)and the elementary substance(B,C,Si)to the compacts are all enhanced the combustion temperature,combustion rate and the highest heating rate,which is according with the theoretical calculation conclusion.The porosity of the products is increased when weight percentage of the additive was elevated,but the open pore proportion is keeping at 95%when the weight percentage of the addition is beyond 12.The characterization of XRD and SEM shows that the product contains TiB2 and TiC,TisSi3,according with the calculated results of the standard formation gibbs free energy.(4)We matched a proper model and equation for the heating layer heat transfer process when the combustion wave is spreaded in the samples.And then measuring thermal conductivity parameter of each sample before reaction,theoretical calculating the influence of metal oxides and nonmetal elemental content on the initiation,and exploring the limitation of the ignition energy and the activation energy of different reaction systemes through the experiment to verify the theoretical model and equation.The results showed that with the elevation of additives,thermal conductivity and thermal diffusivity of samples were all increased,the heating layer thickness and heat flux of heating layer per unit area which are calculated by the equation are decreased,indicates that the samples are much easier to ignited.The measured results of limition ignition energy and the activation energy showed that the promotion of the additives is convenient for ignition,which is consistent with previous theoretical calculation results,and reveals the applicability of the model and equation.